1. Field of the Invention
The present invention relates to the reduction of equivalent circulation density (ECD) in a wellbore. More particularly, the invention relates to the reduction of ECD in a wellbore that is formed while inserting a tubular string that will remain in place in the wellbore as a liner or a casing string. More particularly still, the invention relates to an apparatus and methods to reduce ECD in a wellbore as it is drilled with casing.
2. Description of the Related Art
In the formation of oil and gas wells a borehole is formed in the earth with a drill bit typically mounted at the end of a string of relatively small diameter tubing or drill string. To facilitate the drilling, fluid is circulated through the drill string, out the bit and upward in an annular area between the drill string and the wall of the borehole. The fluid cools the bit and helps remove cuttings. After a predetermined length of borehole is formed, the bit and drill string are removed from the well and larger diameter string called casing or liner is inserted to form the wellbore. The casing is used to line the borehole walls and the annular area between the outer surface of the casing and the borehole is filled with cement to help strengthen the wellbore and aid in isolating sections of the wellbore for hydrocarbon production. In this specification, the terms “borehole” and “wellbore” are used interchangeably and the terms “casing” and “liner” are used interchangeably and relate to a tubular string used to line the walls of a borehole.
The length of borehole formed before it is lined with casing depends largely on pressure developed towards the lower end of the borehole as it is drilled. Because the wellbore is filled with fluid while drilling, a hydrostatic head of pressure is always present and increases with the increased depth of the borehole. Adding to the hydrostatic head is a friction head created by the circulation of the fluid. The combination of hydrostatic and friction heads produces the equivalent circulation density of the fluid. The pressure created by ECD is useful while drilling because it can exceed the pore pressure of formations intersected by the borehole and prevent hydrocarbons from entering the wellbore. However, increased depth of a section of borehole can cause the ECD to exceed a fracture pressure of the formations, forcing the wellbore fluid into the formations and hampering the flow of hydrocarbons into the wellbore after the well is completed. In wells that are drilled in an underbalanced condition, ECD can cause the pressure in the borehole to exceed the pore pressure of the wellbore, making the well over-balanced.
In order to reduce the pressure created by ECD and to increase the length of borehole that can be formed before running in with casing, ECD reduction devices have been used which are designed to be run on drill string and reduce the ECD by adding energy to drilling fluid in the annulus between the drill string and the borehole. Examples include devices that redirect some of the fluid from the drill string out into the annulus and others that have some type of pumping means to add energy to the returning fluid in the annulus. In each instance, the goal is to reduce the effective pressure of the fluid near the bottom of the borehole so that a section of borehole drilled without stopping to run casing can be maximized. An ECD reduction tool and methods for its use is described in co-pending U.S. application Ser. No. 10/156,722 and that specification, filed May 28, 2002 is incorporated herein in its entirety. Additional examples of ECD tools are discussed in Publication No. PCT/GB00/00642 and that publication is also incorporated herein by reference it its entirety.
Drilling with casing is a method of forming a borehole with a drill bit attached to the same string of tubulars that will line the borehole. In other words, rather than run a drill bit on smaller diameter drill string, the bit is run at the end of larger diameter tubing or casing that will remain in the wellbore and be cemented therein. The advantages of drilling with casing are obvious. Because the same string of tubulars transports the bit as lines the borehole, no separate trip into the wellbore is necessary between the forming of the borehole and the lining of the borehole. Drilling with casing is especially useful in certain situations where an operator wants to drill and line a borehole as quickly as possible to minimize the time the borehole remains unlined and subject to collapse or the effects of pressure anomalies. For example, when forming a sub-sea borehole, the initial length of borehole extending from the ocean floor is much more subject to cave in or collapse as the subsequent sections of borehole. Sections of a borehole that intersect areas of high pressure can lead to damage of the borehole between the time the borehole is formed and when it is lined. An area of exceptionally low pressure will drain expensive drilling fluid from the wellbore between the time it is intersected and when the borehole is lined. In each of these instances, the problems can be eliminated or their effects reduced by drilling with casing. Various methods and apparatus for drilling with casing are disclosed in co-pending application Ser. No. 09/848,900 filed May 4, 2001 and that specification is incorporated herein in its entirety.
The challenges and problems associated with drilling with casing are as obvious as the advantages. For example, the string of casing must fit within any preexisting casing already in the wellbore. Because a string of casing transporting the drill bit is left to line the borehole, there is no opportunity to retrieve the bit in the conventional manner. Drill bits made of drillable material, two-piece drill bits and bits integrally formed at the end of casing string have been used to overcome the problems. For example, a two-piece bit has an outer portion with a diameter exceeding the diameter of the casing string. When the borehole is formed, the outer portion is disconnected from an inner portion that can be retrieved to the surface of the well. Typically, a mud motor is used near the end of the liner string to rotate the bit as the connection between the pieces of casing are not designed to withstand the tortuous forces associated with rotary drilling. In this manner, the casing string can be rotated at a moderate speed at the surface as it is inserted and the bit rotates at a much faster speed due to the fluid-powered mud motor.
Equivalent circulating density is as big a factor when drilling with casing as when drilling with conventional drill string because fluid must still be circulated while the borehole is being formed. Because the diameter of the casing is so near the internal diameter of the borehole, conventional ECD reduction techniques are problematic. For example, using a fluid powered pump to add energy to the returning fluid in the annulus between the casing and the borehole is more challenging because there is so little space in the annulus for the blades of a pump. More problematic, any fluid pump/impeller device must operate in the interior of the casing string and the interior of the casing string must be left free of obstruction prior to cementing. Additionally, redirecting fluid from the interior to the exterior of the casing to reduce ECD necessarily requires a fluid path between the interior and exterior of the casing. However, the casing string, to be properly cemented in place must be free of fluid paths between its interior and exterior.
There is a need therefore for a method and apparatus that permits drilling with casing while reducing ECD developed during the drilling process. There is a further need for a method and an apparatus of drilling with casing that leaves the interior of the casing free of obstruction after the borehole is formed. There is yet a further need for a method and apparatus that leaves the walls of the casing ready for cementing after the borehole is formed.